Circuit for determining carrier frequencies of frequency modulated signals



I IIJIIIW I 1 gr 1949- M. c. PEASE 2,478,311

CIRCUIT FOR DETERMINING CARRIER FREQUENCIES J :L-F" OF FREQUENCYMODULATED SIGNALS Filed March 4, 1946 I; '4

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I2 H. P. BOLOMETER I I 1 H FILTER CIRCUIT LOW PASS I I MIXER I? I IFILTER L. P. BOLOMETER FILTER cmcun I I I I ISOLATION 7 l8 I9 20 I STAGE|o- RECEIVER I I LOCAL -oscu AToR- T I I ll-TRANSM\TTER I I ISOLATION 23I I STAGE I I FIXED x IOSCILLATOR Ml ER I I I I as I I HIGH PASS 2FILTER Y I I 27-0UTPUT I l I D O- I- D O INVENTOR MARSHALL C. PEASE By vFREQUENCY ATTORNEY Patented Aug. 9, 1949 UNITED STATES PATENT OFFICECIRCUIT FOR DETERMINING CARRIER FRE- QUENCIES OF FREQUENCY MODULATEDSIGNALS Marshall 0. Pease, Boston, Mass, assignor to the United Statesof America as represented by the Secretary of War 2 Claims.

This invention relates generally to electrical apparatus and moreparticularly to apparatus for producing interference upon (jamming) acommunication channel.

A method of j ariarin ing a frequency modulated signal is to repea' ysweep the jamming signal through a frequency spectrum which includes thecarrier frequency of the frequency modulated signal. This sometimesproves unsatisfactory, since there may be other communication channelsin the same frequency spectrum which should not be interfered with.

Another method of jamming a frequency modulated signal channel is togenerate a signal of the same carrier frequency as that of the channelto be jammed. This minimizes interference to other channels and improvesthe effectiveness of the jamming apparatus. This method, however, posesdifiiculties in determining and reproducing the carrier frequency.

It is, therefore, an object of this invention to provide means fordetermining the carrier frequency of a received frequency modulatedsignal.

Another object is to provide means for generating a jamming signal atthe carrier frequency of a selected frequency modulated signal.

Other objects, features, and advantages of the invention will suggestthemselves to those skilled in the art and will be apparent from thefollowing description of the invention taken in connection with theaccompanying drawing in which:

Fig. 1 is a block diagram of the frequency determining and generatingcircuits; and

Fig. 2 is a graph of the transmission curves of the matched filtersshown in the frequency determining circuit of Fig. 1.

Referring now more particularly to Fig. 1, the circuit components arefor purposes of description divided into two groups which are shownenclosed by dotted line blocks. That group enclosed by the dotted lineblock It] comprises the circuit in which the carrier frequency of thesignal to be jammed is determined, hereinafter termed the frequencydetermining circuit; and that portion enclosed by the dotted line blockH comprises the circuit for generating a signal at the said carrierfrequency. A variable frequency local oscillator 22 is common to bothcircuits as shown, and is connected to mixers l3 and 24 throughisolating stages 20 and 23, respectively. There is also connected tomixer IS an input circuit from an antenna 12. The output of mixer l3passes through a low-pass filter 14 to two filters l5 and I8 havingtheir input circuits connected in parallel. The outputs of filters l5and I8 are connected to direct reading meters I1 and 20 throughbolometer circuits l6 and I9, respectively. A fixed frequency oscillator25 is connected to a mixer 24, and the output of the mixer 24 passesthrough a high-pass filter 26 to an output terminal 21 as shown.

In operation, the frequency modulated signal received at the antenna I2is fed into the mixer l3 where it is combined with the signal generatedby local oscillator 22. The output of mixer I3 passes through thelow-pass filter I4, which is designed to pass only the difierencefrequency of the incoming signal and the local oscillator, into the pairof filters Ni and IS. The energy passed by each of these filters l5 andI8 is measured by means of the bolometer circuits l6 and I9 and theoutput meters l1 and 20, respectively. Filters l5 and [8 are high-pass.and low-pass filters, respectively, having transmission characteristicsinterrelated as indicated in Fig. 2, and are hereinafter referred to asmatched filters. The frequency of local oscillator 22 is adjusted untilthe two output meters indicate that the same amount of energy is beingpassed by the high-pass filter I5 as is passed by the low-pass filter[8. Thus, the local oscillator 22 is then so tuned that the differencein frequency between the local oscillator and the carrier of theincoming frequency modulated signal is equal to the common cutofffrequency in of the matched filters I5 and [8.

In the frequency generating or transmitter circuit shown in dotted lineblock H, the fixed frequency oscillator 25 is pre-set to operate at thecutoff frequency of the matched high-pass and low-pass filters l5 andI8. The combination of the local oscillator signal with the fixedoscillator signal in mixer 24 produces a jamming signal having thecarrier frequency of the incoming signal. The output of mixer 24 passesthrough a high-pass filter 26 designed to transmit only the sumfrequency which results from the addition of the fixed oscillatorfrequency and the frequency of the local oscillator 22. The frequency ofthe output signal from filter 26 is thus the same as the carrierfrequency of the received frequency modulated signal.

As an example of operation, we may assume that the apparatus is tooperate over a frequency range between 175 and 225 megacycles. Thelowpass filter I A is then designed to have a cutoff frequency ofmegacycles. The matched filters l5 and [8 are designed to have a commoncut- Qlf frequency of Z5 megacycles. The local oscillator 22 is adaptedto be tuned between 100 and 150 megacycles. For an incoming frequencymodulated signal having a carrier frequency of 200 megacycles, when thelocal oscillator 22 is tuned to 125 megacycles the energy passed by thehigh-pass filter I will be equal to that passed by the low-pass filterl8, and the two output meters will give identical readings. Thefrequency of the incoming signal may be determined by adding the cutofffrequency of matched filters l5 and [8 to the frequency of localoscillator 22. Thus, in this instance, the carrier frequency of theincoming signal so determined is 200 megacycles. Local oscillator 22may, if desired, be calibrated, although this does not alter or increasethe effectiveness of the described jamming apparatus in generating asignal at the carrier frequency of the incoming signal.

In the transmitter circuit, the fixed frequency oscillator 25 generatesa signal of the same frequency as the cutoff frequency of matchedfilters I5 and I8, which in this example is 75 megacycles. This signalcombines with that of the local oscillator in mixer 24, and, from thehigh-pass filter 26 connected thereto, there is obtained a jammingsignal having, in this instance, a carrier frequency of 200 megacycleswhich is the same as that of the received signal.

It will be seen that the invention herein described provides a simpleand effective means for determining the carrier frequency of a frequencymodulated signal and for simultaneously generating a jamming signalhaving the same frequency.

While there has been described what is at present considered to be apreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope and spirit of the invention asset forth in the appended claims.

What is claimed is:

1. A circuit for determining the carrier frequency of a receivedfrequency modulated signal and for transmitting a signal at said carrierfrequency, said circuit comprising a carrier frequency determiningmeans, a carrier frequency generating means, and a variable frequencylocal oscillator; said carrier frequency determining means comprising anantenna, an input mixer connected to said antenna and to said lodaloscillator, the output of said mixer containing the difference frequencybetween said received signal and the output of said local oscillator, ahigh pass filter and a low pass filter connected in parallel to receivesaid difference frequency, said filters being matched and having acommon predetermined cutoff frequency, and energy indicating meansconnected to the output of each of said filters for indicating when thedifference frequency between said variable frequency local oscillatorand said received signal is equal to said predetermined cut-offfrequency; said frequency generating means comprising a fixed oscillatoroperating at said cutoff frequency, an output mixer connected to saidlocal and fixed oscillators, the output of said output mixer containingthe sum frequency of said oscillators, said sum frequency being equal tothe frequency of said received signal when said means indicate equalvalues.

2. A method for jammin an undetermined frequency modulated signalcomprising receiving said signal, generating a local signal,heterodyning said signals, deriving a modulated frequency differencesignal therefrom, filtering all frequencies contained in said differencesignal above a predetermined frequency into a first channel, filteringall frequencies contained in said difference frequency signal below saidpredetermined frequency into a second channel, measuring the outputs ofeach channel, varying the frequency of the locally generated signaluntil the outputs of said channels are equal, generating a second localsignal at said predetermined frequency, heterodyning both of said localsignals, deriving a sum frequency therefrom, and transmitting said sumfrequency.

MARSHALL C. PEASE.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS

